z =0 is the face of the lead glass at phtocupling suface with phototubes.

The data points are https://drupal.star.bnl.gov/STAR/blog/bsumma/geant-cerenkov-summary  (Slide-7 from Len's work)
The data poits are taken the from the red points (Standard Attenuation)

The expnential Fit with 100% light collecting efficiency at z=0.

For Small Towers : 

parameter [0] = -2.02547e-08 
parameter[1]  = -0.0399546

For Large Towers : 

parameter [0] =  1.09412e-08
parameter[1]  =  -2.61958e-02

Due to diffrent widths in large and in small towers, the z distaance travelled in a single reflection in large cell is increased by a factor 5.8/3.81. And in in first approximation it resonable to set the same attinuation for large tower same that of small towers with z distance scaled by a factor 5.8/3.81.